This invention relates to a packet scheduler and method therefor, and in particular to a packet scheduler for a cellular communication system for mobile communication.
In a cellular communication system each of the remote terminals (typically mobile stations) communicates with typically a fixed base station. Communication from the remote terminal to the base station is known as uplink and communication from the base station to the remote terminal is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of remote terminals 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a remote terminal moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the remote terminal and the base station of the first cell, to being between the remote terminal and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a remote terminal is routed through the fixed network to the destination specific for this call. If the call is between two remote terminals of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other remote terminal currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a remote terminal and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the remote terminals and the base stations. This spectrum must be shared between all remote terminals simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of remote terminals can be accommodated in the same wideband spectrum by allocating different codes for different remote terminals. Codes are chosen to minimise the interference caused between remote terminals typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in xe2x80x98Spread Spectrum CDMA Systems for Wireless Communicationsxe2x80x99, Glisic and Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Traditional traffic in mobile cellular communication systems has been circuit switched voice data where a permanent link is set up between the communicating parties. In the future it is envisaged that data communication will increase substantially and typically the requirements for a remote terminal to transmit data will not be continuous but will be at irregular intervals. Consequently it is inefficient to have a continuous link setup between users and instead a significant increase in packet based data traffic is expected, where the transmitting remote terminal seeks to transmit the data in discrete data packets when necessary. An example of a packet based system is General Packet Radio Service (GPRS) introduced to the Global System for Mobile communication (GSM). Further details on data packet systems can be found in xe2x80x98Understanding data communications: from fundamentals to networking, 2nd ed.xe2x80x99, John Wiley publishers, author Gilbert Held, 1997, ISBN 0-471-96820-X.
In a packet based system where a high number of remote terminals may require resources for packet transmissions at unknown and irregular intervals it is important for optimal utilisation of the limited resource to schedule the order and time for transmission of the individual packets. This becomes even more important when different data packets have different requirements with respect to delay, bit error rate etc. Therefore most packet based systems contain schedulers which control when the individual data packets are transmitted and therefore share the available resource, whether time-slots in a TDMA system or power and codes in a CDMA system. An introduction to schedulers can be found in xe2x80x98Service discipline for guaranteed performance service in packet-switching networksxe2x80x99, Hui Zhang, Proceedings of the IEEE, volume 83, no. 10, October 1995.
However, known schedulers have been optimised for different environments than CDMA systems. For example, scheduling algorithms used for GPRS are optimised for a Time Division Multiple Access (TDMA) system and therefore not optimal for CDMA systems where codes and power must be shared.
The inventors of the current invention have realised that conventional approaches for scheduling of data packets can be improved in a CDMA system by considering parameters specific to CDMA when scheduling.
Accordingly there is provided a method of packet scheduling for a Code Division Multiple Access cellular communication system having a number of cells, each cell having a base station serving at least one remote terminal, the method comprising the steps of: selecting a candidate set of packets: based on the candidate set calculating transmit powers required to transmit the packets; modifying the candidate set in response to the calculated transmit powers; and scheduling the packets of the candidate set for transmission.
By scheduling packets in response to an calculation of transmit powers required to support a given candidate set of packets, the invention provides an improvement in the resource utilisation (the fraction of the available resource which is exploited by the scheduler whilst meeting the agreed quality of service for the supported circuit and packet connections) or capacity, whilst at the same time running at low complexity.
Preferably the step of modifying the candidate cells and the calculation of transmit powers are iterated until a final candidate set has been determined, where the final set is preferably the first set for which all packets awaiting transmission have been in the candidate set at least once.
According to a second aspect of the invention there is provided a packet scheduler for a Code Division Multiple Access cellular communication system having a number of cells each cell having a base station serving at least one remote terminal, the scheduler comprising: means for selecting a candidate set of packets; means for calculating transmit powers required by remote terminals to transmit the packets based on the candidate set; means for modifying the candidate set in response to the calculated transmit powers; and means for scheduling the packets of the candidate set for transmission.